Agriculture Reference
In-Depth Information
than when grown on combined nitrogen, and in the worst the nodules fix no nitrogen
and are thus parasitic as far as the plant is concerned although the bacteria can obtain
considerable benefit. Provorov and Tikhonovich (2003) gave evidence that crop species
that have been bred for many years (e.g. peas, soybeans lucerne), often on soils contain-
ing combined nitrogen, usually contain less nitrogen when grown symbiotically than
when grown on combined nitrogen. On the other hand, 'young' crops such as some
vetches (species of
Vi c ia
) show the reverse. Kiers et al. (2007) looked at the effects of
domestication on the effectivess of soybean nodules and found that 'legume defences
against poor quality inoculants have apparently worsened under decades of artificial
selection' (on fertilised soils).
Measurement of nitrogen in the plant can show ineffectiveness in nodules, evenwhen
they appear to be normal in structure, as recently found in the model legume
Medicago
truncatula
when grownwith
Sinorhizobiummeliloti
rather than
S. medicae
(Terpolilli et al.,
2008; see also photograph in Sprent & James, 2008). In more extreme cases the number
of infected cells in the nodule may be reduced (Gross et al., 2002), and nodules may
have less haemoglobin or even no pink coloration at all (Sprent, 2001).There is a close
correlation between different forms of haemoglobin and induction of genes necessary
for nitrogen fixation (Downie, 2005; Ott et al., 2005). In some cases ineffectivity is
accompanied by production of many small nodules; in others nodules are enlarged
(Sprent, 2001). In all cases, the bacteria seem able to enter the plant in the normal way,
with things going wrong at any stage thereafter. Until the details of normal nodule
development are better understood, the exact process that is disrupted when nodules
are ineffective will remain obscure. However, ineffectiveness is a very real problem,
especially in the tropics (Sprent, 2003) and in large legume genera that can nodulate
with a variety of rhizobia (e.g. both African and Australian acacias (Odee et al., 2002,
Thrall et al. 2000). Howieson and McInnes (2001), in discussing this problem, also
suggest ways in which it may be improved by plant breeding and rhizobial selection.
Examples of ineffective nodulation are given in Plate 5.4.
5.10 The bacteria within the nodule - control by the bacteria,
plant or both?
Themost intensive studies in recent years have been largely conducted on single strains
of rhizobia with single host genotypes, as with the model legumes, or in more complex
experiments with several such pairs of organisms under varying environments. As far
as the plant is concerned, the major requirement is to use carbon resources carefully
to give the maximum return in terms of nitrogen fixed. As far as the bacteria are
concerned, the need is tomaintain reproductive capacity for organisms living in diverse
environments. In the last few years, experimental studies on these problems have been
supplemented by theoretical considerations. Further, as knowledge of legumes outside
the usual run of crop and model species increases, it is apparent that there is great
flexibility in the system. Some of the current ideas and dilemmas are considered in this
section.
